A lithium-ion battery having a high capacity density and a high mass density is mounted not only on a mobile device such a mobile phone or a digital camera but also on an electric bicycle, an electric motor cycle, an electric vehicle, and the like. Further, with diversification of battery usage, demands for a reduction in weight of the battery and high flexibility in design have increased.
As a battery covering member, a metal casing or a film covering material obtained by stacking an aluminum foil or the like and a synthetic resin film is used. As a battery that meets the above requirements, a film-covered battery using the film covering material which is light in weight and high in design freedom degree is much in demand.
The film-covered battery has a configuration in which a battery element having a positive electrode and a negative electrode is covered and sealed with the film covering material such that a positive electrode terminal and a negative electrode terminal are drawn outside the film covering material. The battery covering member has a role of preventing leakage of electrolyte inside the battery and infiltration of water into an inside of the battery, and the sealing portion is responsible for reliability of the battery and has thus been variously devised.
For example, in Patent Document 1, a resin mass 32 is provided at an inner end of a housing portion 6a of a battery element 6 of an agglomeration/sealing portion 19 which is a welding portion of a metal laminated resin film casing, as illustrated in FIG. 10. As a result, a joint interface has a curved surface, allowing stress to be dispersed to both ends of the resin mass 32, which enhances sealability of the battery.
Further, in Patent Document 2, there is provided a portion where welding is not performed at an inner side of a heat sealing portion of the film covering material. This prevents occurrence of a cut portion of a resin protrusion portion to prevent decrease in peeling strength.